1. Field of the Invention
This invention relates to novel luminescent materials, and more particularly, this invention relates to a novel class of luminescent materials of metal chelates with mixed ligands and organic electroluminescent (EL) devices made from such materials.
2. Description of the Related Art
During the last decade, an explosive growth of activity in the area of organic electroluminescence has occurred in both academia and industry, stimulated by the promise of light-emitting plastics for the fabrication of large, flexible, inexpensive and efficient screens to be used in different applications. A great deal of work has been carried out by physicists and materials scientists concerned with the preparation of different device structures and with the use of different techniques for device manufacture. The design of luminescent materials for use in LED devices is as critical to device performance as the process of constructing the device itself. Processability, purity, thermal and oxidative stability, color of emission, luminance efficiency, balance of charge carrier mobility, and others are among many important materials properties required for a system to be viable in commercial LED device applications. As a result, numerous organic compounds intended for OLEDs have been developed in the past decade.
Organometallic complex luminescent materials for use in organic electroluminescent (EL) devices have been reported. Commonly-assigned U.S. Pat. No. 4,720,432 issued Jan. 19, 1988, discloses the efficient organic EL devices using the organometallic complex, metal chelates of 8-quinolinolato ligands, as a luminescent medium. Tris(8-hydroxyquinoline) aluminum (Alq3) is a stable metal chelate that can be sublimed to yield amorphous thin films and stands as one of the most successful organic materials used in OLEDs. And many derivatives of Alq3 have been investigated as emitter materials, including substitution of the metal ion with other trivalent metals (M+3=Ga, In, and Sc) and substitution of the 8-quinolinol ligand (substituent=F, Cl, CN, and alkyl groups). However, most of the previous work on metal-chelate EL complexes have focused on system with one kind of ligand such as Alq3, and much less attention has been directed toward metal systems containing two different ligands. This type of EL material is expected to be amorphous due to the asymmetric structure of the molecular constituents.
In our group, we focused on metal(III) complexes based on tridentate schiff-base ligands, which have proven to be high efficient luminescent materials for OLEDs. Some preliminary communication of this work has been reported on (salicylidene-o-aminophenolato)(8-quinolinato) aluminum (Al(Saph)-q) and its derivatives for the first time by Y. Shao and Y. Qiu et al. in Adv. Mater. Opt. Electro. 2000, 10, 285, and U.S. Pat. No. 6, 410,766 issued in Jun. 25, 2002. It has been proven that the introduction of the tridentate schiff-base ligand could greatly improve the thermal stability and film-forming capability of aluminum complexes for organic EL.